Strength members, reinforcing members, and structural members made of metal are used in automobiles and various types of machines. These members are required to have a high strength, a light weight, and a small size. From in the past, these members have been manufactured by working methods such as welding of press-formed parts, and punching or forging of thick plates. However, it is extremely difficult to further decrease the weight and size of members manufactured by these manufacturing methods. For example, when manufacturing welded parts by partially overlapping two panels formed by press working and welding them, it is necessary to form portions of excess thickness referred to as flanges on the edges of the panels, and as a result, the weight of the welded parts unavoidably increases by an amount corresponding to the excess thickness.
A working method referred to as hydroforming (see, for example, Non-patent Document 1) forms a tube into a complicated shape by introducing a working fluid at a high pressure into the interior of a pipe which is a material to be worked disposed inside a mold and carrying out deformation by expanding the pipe so that the outer surface of the pipe conforms to the inner surface of the mold. Parts having a complicated shape are integrally formed by hydroforming without the need to form flanges. In recent years, hydroforming has been actively applied to automotive parts with the objective of decreasing the weight of automotive parts.
Hydroforming is a type of cold working. Therefore, forming a material to be worked having a high strength such as at least 780 MPa into an automotive part having a complicated shape is difficult due to inadequate ductility of the material to be worked. As hydroforming generally requires three manufacturing steps, i.e., bending, preforming, and hydroforming, it is relatively complicated. Furthermore, a hydroforming apparatus is large and relatively expensive.
The present applicant disclosed a working apparatus in Patent Document 1. FIG. 7 is an explanatory view schematically showing this working apparatus 0.
The working apparatus 0 manufactures a bent member using a metal material 1 as a material to be worked by the following steps.
(a) A support means 2 supports the metal material 1 so that it can move in its axial direction.
(b) A feed device 3 feeds the metal member 1 which is supported by the support means 2 from the upstream side to the downstream side while the metal member 1 undergoes bending on the downstream side of the support means 2.
(c) Bending is carried out in the following manner. An induction heating coil 5 disposed downstream of the support means 2 locally, rapidly heats the metal member 1 to a temperature range in which quench hardening is possible. A cooling device 6 (such as a water cooling device) disposed immediately downstream of the induction heating coil 5 rapidly cools the metal member 1. A movable roller die 4 has at least one set of roll pairs 4a which can support the metal member 1 while feeding it. The movable roller die 4 is disposed downstream of the cooling device 6. By varying its position two-dimensionally or three-dimensionally, the movable roller die imparts a bending moment to the heated portion of the metal member 1.
Namely, a bent member is manufactured by the working apparatus 0 through the following steps.
(I) An elongated metal material 1 which has a hollow closed cross-sectional shape and is constituted by a single piece in the lengthwise direction is worked by a pair of rolls to form an elongated metal intermediate member having a flat, hollow, closed cross-sectional shape with a pair of opposing longer sides.
(II) The feed device 3 performs relative feeding of the intermediate member in its lengthwise direction.
(III) The support means 2 supports the intermediate member being fed at a first position.
(IV) The induction heating coil 5 locally heats the intermediate member being fed at a second position downstream of the first position in the feed direction of the intermediate member.
(V) The cooling device 6 cools the heated portion of the intermediate member being fed at a third position downstream of the second position in the feed direction of the intermediate member.
(VI) A bending moment is applied to the heated portion of the intermediate member by two-dimensionally or three-dimensionally varying the position of the movable roller die 4 which supports the intermediate member being fed in a region downstream of the third position in the feed direction of the intermediate member.
The working apparatus 0 can perform shaping of a one piece automotive part having a high strength such as at least 780 MPa and a complicated shape by simple steps using relatively inexpensive forming equipment. In this manner, a bent member having a high stiffness is manufactured by the working apparatus 0.